Angularly resolved scatterometer, inspection method, lithographic apparatus, lithographic processing cell device manufacturing method and alignment sensor

ABSTRACT

An angularly resolved scatterometer uses a broadband radiation source and an acousto-optical tunable filter to select one or more narrowband components from the broadband beam emitted by the source for use in measurements. A feedback loop can be used to control the intensity of the selected narrowband components to reduce noise.

FIELD

The present invention relates to methods of inspection usable, forexample, in the manufacture of devices by lithographic techniques and tomethods of manufacturing devices using lithographic techniques.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at once, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to monitor the lithographic process, it is desirable to measureparameters of the patterned substrate, for example the overlay errorbetween successive layers formed in or on it. There are varioustechniques for making measurements of the microscopic structures formedin lithographic processes, including the use of scanning electronmicroscopes and various specialized tools. One form of specializedinspection tool is a scatterometer in which a beam of radiation isdirected onto a target on the surface of the substrate and properties ofthe scattered or reflected beam are measured. By comparing theproperties of the beam before and after it has been reflected orscattered by the substrate, the properties of the substrate can bedetermined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withknown substrate properties. Two main types of scatterometer are known.Spectroscopic scatterometers direct a broadband radiation beam onto thesubstrate and measure the spectrum (intensity as a function ofwavelength) of the radiation scattered into a particular narrow angularrange. Angularly resolved scatterometers use a monochromatic radiationbeam and measure the intensity of the scattered radiation as a functionof angle.

Many types of metrology (inspection) devices produce results that aredependent on the wavelength of the radiation used, for example becauselayers on the substrate being inspected have wavelength dependentoptical properties or because layer thicknesses or structural pitchesdifferently diffract the radiation used. Thus inspection devices ofvarious types use either multiple monochromatic sources (e.g. lasers)providing outputs of different wavelengths or broadband sources andselectively insertable filters to enable measurements at differentwavelengths to be performed either simultaneously or selectively.

However, coupling multiple laser sources into a single optical system ofan inspection device often requires expensive optical multiplexingand/or demultiplexing devices and may be sensitive to alignment of thesources and other optical components. Swapping filters into and out ofthe inspection beam can be time consuming, reducing throughput of theinspection device when multiple measurements at different wavelengthsneed to be made.

An ellipsometer using a broadband source and an acousto-optical filterfor fast wavelength selection is disclosed in WO 95/17662.

SUMMARY

It is desirable to provide an inspection device that can makemeasurements at multiple different wavelengths and that does not sufferfrom disadvantages of the prior art.

According to an embodiment of the invention, there is provided anangularly resolved scatterometer configured to determine a value relatedto a parameter of a target pattern printed on a substrate by alithographic process used to manufacture a device layer on a substrate,the apparatus including a broadband radiation source arranged to emit afirst beam of radiation having a first wavelength range; anacousto-optical tunable filter including an acousto-optical crystalarranged to receive the first beam of radiation, a transducer coupled tothe acousto-optical filter and arranged to excite acoustic waves thereinand a beam selecting device arranged to select as an output beam one ofa plurality of beams output by the acousto-optical crystal in responseto the first beam and the acoustic waves as a second beam of radiationhaving a second wavelength range, the second wavelength range beingnarrower than the first wavelength range; an optical system including ahigh-NA objective lens arranged to direct the second beam of radiationon to the target pattern and to project radiation reflected or scatteredby the target pattern onto a detector to obtain a scatterometricspectra; and a driver circuit electrically coupled to the transducer andarranged to generate a drive signal therefor, the driver circuit beingadapted to control the frequency of the drive signal so as to controlthe second wavelength range.

According to an embodiment of the invention, there is provided aninspection method to determine a value related to a parameter of atarget pattern printed on a substrate by a lithographic process used tomanufacture a device layer on a substrate, the method including using abroadband radiation source to emit a first beam of radiation having afirst wavelength range; directing the first beam of radiation to anacousto-optical tunable filter including an acousto-optical crystalarranged to receive the first beam of radiation, a transducer coupled tothe acousto-optical filter and arranged to excite acoustic waves thereinand a beam selecting device arranged to select as an output beam one ofa plurality of beams output by the acousto-optical crystal in responseto the first beam and the acoustic waves as a second beam of radiationhaving a second wavelength range, the second wavelength range beingnarrower than the first wavelength range; using an optical systemincluding a high-NA objective lens to direct the second beam ofradiation on to the target pattern and to project radiation reflected orscattered by the target pattern onto a detector to obtain ascatterometric spectra; and providing a drive signal to the transducer,the drive signal having a frequency determined to control the secondwavelength range.

According to an embodiment of the invention, there is provided analignment sensor configured to determine a position of a target patternprinted on a substrate by a lithographic process used to manufacture adevice layer on a substrate, the sensor including a broadband radiationsource arranged to emit a first beam of radiation having a firstwavelength range; an acousto-optical tunable filter including anacousto-optical crystal arranged to receive the first beam of radiation,a transducer coupled to the acousto-optical filter and arranged toexcite acoustic waves therein and a beam selecting device arranged toselect as an output beam one of a plurality of beams output by theacousto-optical crystal in response to the first beam and the acousticwaves as a second beam of radiation having a second wavelength range,the second wavelength range being narrower than the first wavelengthrange; an optical system including a self-referencing interferometer andarranged to direct the second beam of radiation on to the target patternand to project radiation reflected or scattered by the target patternonto a detector; and a driver circuit electrically coupled to thetransducer and arranged to generate a drive signal therefor, the drivercircuit being adapted to control the frequency of the drive signal so asto control the second wavelength range.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus in accordance with an embodimentof the invention;

FIG. 2 depicts a lithographic cell or cluster in accordance with anembodiment of the invention;

FIG. 3 depicts a scatterometer according to an embodiment of theinvention;

FIG. 4 depicts source and filter arrangements of the scatterometer ofFIG. 2;

FIG. 5 depicts a super-continuum laser source usable in embodiments ofthe invention;

FIG. 6 depicts source and filter arrangements of another scatterometeraccording to an embodiment of the invention; and

FIG. 7 depicts an inspection apparatus according to another embodimentof the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g. UV radiation or DUV radiation) a supportstructure (e.g. a mask table) MT constructed to support a patterningdevice (e.g. a mask) MA and connected to a first positioner PMconfigured to accurately position the patterning device in accordancewith certain parameters; a substrate table (e.g. a wafer table) WTconstructed to hold a substrate (e.g. a resist-coated wafer) W andconnected to a second positioner PW configured to accurately positionthe substrate in accordance with certain parameters; and a projectionsystem (e.g. a refractive projection lens system) PL configured toproject a pattern imparted to the radiation beam B by patterning deviceMA onto a target portion C (e.g. including one or more dies) of thesubstrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic-apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice (e.g. mask) MA, the radiation beam B passes through theprojection system PL, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and positionsensor IF (e.g. an interferometric device, linear encoder, 2-D encoderor capacitive sensor), the substrate table WT can be moved accurately,e.g. so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor (which is not explicitly depicted in FIG. 1) can be usedto accurately position the patterning device (e.g. mask) MA with respectto the path of the radiation beam B, e.g. after mechanical retrievalfrom a mask library, or during a scan. In general, movement of thesupport structure (e.g. mask table) MT may be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioner PM. Similarly,movement of the substrate table WT may be realized using a long-strokemodule and a short-stroke module, which form part of the secondpositioner PW. In the case of a stepper (as opposed to a scanner) thesupport structure (e.g. mask table) MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device (e.g.mask) MA and substrate W may be aligned using mask alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the mask MA, the mask alignment marks may belocated between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure (e.g. mask table or patternsupport) MT and the substrate table WT are kept essentially stationary,while an entire pattern imparted to the radiation beam is projected ontoa target portion C at one time (i.e. a single static exposure). Thesubstrate table WT is then shifted in the X and/or Y direction so that adifferent target portion C can be exposed. In step mode, the maximumsize of the exposure field limits the size of the target portion Cimaged in a single static exposure.

2. In scan mode, the support structure (e.g. mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e. a singledynamic exposure). The velocity and direction of the substrate table WTrelative to the support structure (e.g. mask table) MT may be determinedby the (de-)magnification and image reversal characteristics of theprojection system PL. In scan mode, the maximum size of the exposurefield limits the width (in the non-scanning direction) of the targetportion in a single dynamic exposure, whereas the length of the scanningmotion determines the height (in the scanning direction) of the targetportion.

3. In another mode, the support structure (e.g. mask table) MT is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT is moved or scanned while a pattern imparted to theradiation beam is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WT or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. If errors are detected, adjustments may be made to exposures ofsubsequent substrates, especially if the inspection can be done soon andfast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped andreworked—to improve yield—or discarded—thereby avoiding performingexposures on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

An inspection apparatus is used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable most rapid measurements, it is desirable that the inspectionapparatus measure properties in the exposed resist layer immediatelyafter the exposure. However, the latent image in the resist has a verylow contrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not—and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on exposed substrates andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework offaulty substrates but may still provide useful information.

A scatterometer SM2 according to an embodiment of the present inventionis shown in FIG. 3. In this device, the radiation emitted by radiationsource unit 2 (described further below) is focused using lens system 12through polarizer 17, reflected by partially reflected surface 16 and isfocused onto substrate W via a microscope objective lens 15, which has ahigh numerical aperture (NA), preferably at least 0.9 and morepreferably at least 0.95. Immersion scatterometers may even have lenseswith numerical apertures over 1. The reflected radiation then transmitsthrough partially reflective surface 16 into a detector 18 in order tohave the scatter spectrum detected. The detector may be located in theback-projected pupil plane 11, which is at the focal length of the lenssystem 15, however the pupil plane may instead be re-imaged withauxiliary optics (not shown) onto the detector. The pupil plane is aplane in which the radial position of radiation defines the angle ofincidence and the angular position defines azimuth angle of theradiation. The detector is preferably a two-dimensional detector so thata two-dimensional angular scatter spectrum of the substrate target canbe measured. The detector 18 may be, for example, an array of CCD orCMOS sensors, and may use an integration time of, for example, 40milliseconds per frame.

A reference beam is often used for example to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the beam splitter 16 part of it is transmitted through the beamsplitter as a reference beam towards a reference mirror 14. Thereference beam is then projected onto a different part of the samedetector 18.

The detector 18 may measure the intensity of scattered light at a singlewavelength (or narrow wavelength range), the intensity separately atmultiple wavelengths or integrated over a wavelength range. Furthermore,the detector may separately measure the intensity of transversemagnetic- and transverse electric-polarized light and/or the phasedifference between the transverse magnetic- and transverseelectric-polarized light.

The target on substrate W may be a grating, which is printed such thatafter development, the bars are formed of solid resist lines. The barsmay alternatively be etched into the substrate. This pattern can, forexample, be made sensitive to aberrations in the lithographic projectionapparatus, particularly the projection system PL, and illuminationsymmetry and the presence of such aberrations will manifest themselvesin a variation in the printed grating. Accordingly, the scatterometrydata of the printed gratings is used to reconstruct the gratings. Theparameters of the grating, such as line widths and shapes, may be inputto the reconstruction process, performed by processing unit PU, fromknowledge of the printing step and/or other scatterometry processes.Other forms of target may be used to measure other parameters ofstructures on the substrate or the processes used to produce them.

The radiation source unit 2 is shown in more detail in FIG. 4. Thesource includes a broadband source 21, such as for example a xenon lampor a supercontinuum laser, which directs light into an acousto-opticaltunable filter (AOTF) which is used to select a narrow range ofwavelengths from the broadband (white light) output of the source 21 toform the inspection beam in the remainder of the scatterometer, depictedSM in FIG. 4.

The acousto-optical tunable filter (AOTF) includes an acousto-opticalcrystal 22 to which are connected a piezoelectric transducer 23, drivenby a high-frequency driver circuit 24, and an acoustic absorber 25.Transducer 23 creates acoustic waves in the crystal 22 with a wavelengthdetermined by the mechanical properties (speed of sound) of the crystaland driving frequencies. As these waves propagate through the crystalthey form a periodic redistribution of the refractive index of thecrystal due to the alternating expansion and contraction of the crystallattice. This forms a diffraction grating which diffracts the lightpassing through it, although diffraction occurs throughout the region ofinteraction rather than at a single point and only radiation meetingphase and/or momentum matching conditions is diffracted. The net effectis that radiation of a narrow band of wavelengths is diffracted awayfrom the main beam and can be selected by a spatial and/or polarizingfilter 26. The center wavelength of the diffracted beam is dependent onthe driving frequency of the transducer so, it can be controlled withinquite a wide range and very rapidly, dependent on the response time ofthe driver circuit 25, the transducer and the crystal. The intensity ofthe diffracted beam is also controlled by the intensity of the acousticwaves.

Suitable materials which can be used for the acousto-optical crystalinclude: Quartz (SiO2), KDP (KH₂PO₄), Paratellurite or tellurium dioxide(TeO₂), LiNbO3, calomel or mercuric chloride (Hg₂Cl₂), TAS (Ta₃AsSe₃)and Te (tellurium), magnesium fluoride (MgF), and sapphire (aluminumoxide, Al₂O₃). The crystal selected determines the detailed geometry ofthe acousto-optical tunable filter. If a birefringent crystal is used,the filter may also select a particular polarization state.

High frequency drive unit 24 is connected to the control unit CU of thescatterometer which provides a drive signal to cause the transducer toemit acoustic waves of an appropriate frequency to select a narrow bandof wavelengths centered on a desired wavelength, as required for a givenmeasurement. The bandwidth of the transmitted beam is preferably lessthat about 20 nm, less than about 15 nm, less than about 10 nm or lessthan about 5 nm. The exact relationship between frequency of drivesignal and selected wavelength depends on the particular crystalemployed and the geometry of the device. In some cases, by applying adrive signal having two or more components of different frequencies Ω₁to Ω_(n), the filter can be operated to select a plurality of componentseach centered around a different wavelength, which forms a polychromaticbeam that allows a plurality of measurements to be made simultaneously.The intensities of the different frequency components of the drivesignal can be varied to individually control the intensities of thedifferent wavelengths in the polychromatic beam.

An beneficial light source that can be used in an embodiment of thepresent invention is a supercontinuum laser, which is illustrated inFIG. 5. This source includes a pulsed laser source 21 a whose output isfed into a non-linear medium 21 b, e.g. a photonic crystal fiber. Thepulsed source 21 a emits very short pulses, e.g. of femtosecond orpicosecond duration, of a narrow band of wavelengths which are spread bythe non-linear medium 21 b into a broadband beam of radiation. This typeof source can provide a powerful beam with a low etendue and a suitablerange of wavelengths.

An additional benefit of the use of an acousto-optical tunable filter isthat, due to its fast response time, it can be used to reduce noise inthe measurement beam, especially when a supercontinuum laser is used asthe light source. An embodiment of the invention employing this benefitis shown in FIG. 6 and is the same as the embodiment as described below.

In this embodiment of the radiation source unit 2′, after the spatial orpolarizing filter 26, dichroic beamsplitters 31, 32 are positioned inthe beam path. Each dichroic beamsplitter diverts a predeterminedproportion of one component of the output beam of the acousto-opticaltunable filter to a respective detector 33, 34 which measures theintensity of the diverted radiation. The measured intensity value iscompared to a respective setpoint SP1, SP2 and the difference (error) isprovided to a respective controller 35, 36, e.g. aproportional-integral-derivative controller (PID controller), whichprovides a control signal to the driver 24 to control the amplitude ofthe relevant frequency component Ω₁ to Ω_(n) in the drive signal to thetransducer 23. The response time of the control loop 24, 23, 22, 33/34,35/36 can be of the order of microseconds, e.g. 10 μs, 5 μs or 2 μs, soallowing reduction of noise of frequency up to about 40 kHz, which isvery useful. Although two control loops are shown, more than two may beprovided, each controlling an output beam of different wavelength. Itshould be noted that if the noise in the beam output by the radiationsource 21 is relatively constant over a wavelength range encompassingseveral different components of the output beam, one control loop may beused to control the amplitude of more than one frequency component inthe drive signal. In the limit, a single control loop may control theamplitude of all frequency components of the drive signal and hence theintensities of all components of the output beam.

Another embodiment of the invention is depicted in FIG. 7. This includesan alignment system 40 of the type described in EP-A-1,148,390, whichdocument is hereby incorporated by reference, and uses a compactself-referencing interferometer 42 and a beam splitter 41 to generatetwo overlapping images of a target on substrate W, rotated over +90° and−90°, which are then made to interfere in a pupil plane. An opticalsystem and spatial filter selects and separates the first order beamsand re-images them on a detector 43 whose output is processed by asignal analyzer 44 to provide the required measurement. In a variant ofthis type of metrology device which can also be employed in the presentinvention, the detector can be placed in the pupil plane. Furtherdetails are given in EP-A-1 372 040, which document is herebyincorporated by reference. The light source 2′, which is the same as thelight source 2′ described above, is used to provide a beam of radiationof the desired wavelength or wavelengths that has a stable intensity andreduced noise, which is ideal for this type of alignment sensor.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. An angularly resolved scatterometer configured to determine a value related to a parameter of a target pattern printed on a substrate by a lithographic process used to manufacture a device layer on the substrate, the apparatus comprising: a broadband radiation source arranged to emit a first beam of radiation having a first wavelength range; an acousto-optical tunable filter comprising an acousto-optical crystal arranged to receive the first beam of radiation, a transducer coupled to the acousto-optical filter and arranged to excite acoustic waves therein, and a beam selecting device arranged to select as an output beam one of a plurality of beams, output by the acousto-optical crystal in response to the first beam and the acoustic waves, as a second beam of radiation having a second wavelength range, the second wavelength range being narrower than the first wavelength range; an optical system comprising a high-NA objective lens arranged to direct the second beam of radiation onto the target pattern and to project radiation reflected or scattered by the target pattern onto a detector to obtain a scatterometric spectrum; and a driver circuit electrically coupled to the transducer and arranged to generate a drive signal therefor, the driver circuit being adapted to control a frequency of the drive signal so as to control the second wavelength range.
 2. An angularly resolved scatterometer according to claim 1, wherein the second beam of radiation has a bandwidth of less than about 20 nm.
 3. An angularly resolved scatterometer according to claim 2, wherein the second beam of radiation has a bandwidth of less than about less than about 10 nm.
 4. An angularly resolved scatterometer according to claim 3, wherein the second beam of radiation has a bandwidth of less than about 5 nm.
 5. An angularly resolved scatterometer according to claim 1, further comprising a detector; and a beam splitter arranged to divert a predetermined portion of the second beam of radiation to the detector, wherein the detector is arranged to detect an intensity of the diverted portion of the second beam of radiation and to generate an intensity measurement signal, the driver circuit being responsive to the intensity measurement signal to control an amplitude of the drive signal and thereby an amplitude of the second beam of radiation.
 6. An angularly resolved scatterometer according to claim 1, wherein the drive circuit is arranged to generate a drive signal having a plurality of different frequency components, wherein the acousto-optical tunable filter is configured to output simultaneously a plurality of second beams of radiation having respectively different wavelength ranges.
 7. An angularly resolved scatterometer according to claim 6, further comprising a detector; and a beam splitter arranged to divert a predetermined portion of the second beams of radiation to the detector, wherein the detector is arranged to detect the intensity of the diverted portion of the second beams of radiation and to generate an intensity measurement signal, the driver circuit being responsive to the intensity measurement signal to control an amplitude of the drive signal and thereby an amplitude of the second beam.
 8. An angularly resolved scatterometer according to claim 7, wherein the beam splitter is a dichroic beam splitter arranged to divert a predetermined portion of a specific one of the second beams of radiation to the detector.
 9. An angularly resolved scatterometer according to claim 6, further comprising a plurality of detectors; and a plurality of dichroic beam splitters each arranged to divert a predetermined portion of a respective one of the second beams of radiation to a respective one of the detectors, wherein each of the detectors is arranged to detect an intensity of the diverted portion of the respective second beam and to generate a respective intensity measurement signal, the driver circuit being responsive to the intensity measurement signal to control the amplitude of the respective frequency component of the drive signal and thereby the amplitude of the respective second beam of radiation.
 10. An angularly resolved scatterometer according to claim 1, wherein the broadband radiation source is a supercontinuum laser.
 11. An angularly resolved scatterometer according to claim 1, wherein the broadband radiation source is a xenon lamp.
 12. A lithographic apparatus comprising: an illumination optical system arranged to illuminate a pattern; a projection optical system arranged to project an image of the pattern onto a substrate; and an angularly resolved scatterometer including a broadband radiation source arranged to emit a first beam of radiation having a first wavelength range; an acousto-optical tunable filter comprising an acousto-optical crystal arranged to receive the first beam of radiation, a transducer coupled to the acousto-optical filter and arranged to excite acoustic waves therein, and a beam selecting device arranged to select as an output beam one of a plurality of beams, output by the acousto-optical crystal in response to the first beam and the acoustic waves, as a second beam of radiation having a second wavelength range, the second wavelength range being narrower than the first wavelength range; an optical system comprising a high-NA objective lens arranged to direct the second beam of radiation onto a target pattern and to project radiation reflected or scattered by the target pattern onto a detector to obtain a scatterometric spectrum; and a driver circuit electrically coupled to the transducer and arranged to generate a drive signal therefor, the driver circuit being adapted to control the frequency of the drive signal so as to control the second wavelength range.
 13. A lithographic cell comprising: a coater arranged to coat substrates with a radiation sensitive layer; a lithographic apparatus arranged to expose images onto the radiation sensitive layer of the substrates coated by the coater; a developer arranged to develop images exposed by the lithographic apparatus; and an angularly resolved scatterometer according to claim
 1. 14. An inspection method to determine a value related to a parameter of a target pattern printed on a substrate by a lithographic process used to manufacture a device layer on the substrate, the method comprising: emitting a first beam of radiation having a first wavelength range using a broadband radiation source; directing the first beam of radiation to an acousto-optical tunable filter comprising an acousto-optical crystal arranged to receive the first beam of radiation, a transducer coupled to the acousto-optical filter and arranged to excite acoustic waves therein and a beam selecting device arranged to select as an output beam one of a plurality of beams, output by the acousto-optical crystal in response to the first beam and the acoustic waves, as a second beam of radiation having a second wavelength range, the second wavelength range being narrower than the first wavelength range; directing the second beam of radiation onto the target pattern using an optical system comprising a high-NA objective lens and projecting radiation reflected or scattered by the target pattern onto a detector to obtain a scatterometric spectrum; and providing a drive signal to the transducer, the drive signal having a frequency determined to control the second wavelength range.
 15. A device manufacturing method comprising: forming a pattern on a substrate using a lithographic apparatus; and determining a value related to a parameter of the pattern formed on the substrate by: emitting a first beam of radiation having a first wavelength range using a broadband radiation source; directing the first beam of radiation to an acousto-optical tunable filter comprising an acousto-optical crystal arranged to receive the first beam of radiation, a transducer coupled to the acousto-optical filter and arranged to excite acoustic waves therein and a beam selecting device arranged to select as an output beam one of a plurality of beams, output by the acousto-optical crystal in response to the first beam and the acoustic waves, as a second beam of radiation having a second wavelength range, the second wavelength range being narrower than the first wavelength range; directing the second beam of radiation onto the target pattern using an optical system comprising a high-NA objective lens and projecting radiation reflected or scattered by the target pattern onto a detector to obtain a scatterometric spectrum; providing a drive signal to the transducer, the drive signal having a frequency determined to control the second wavelength range.
 16. An alignment sensor configured to determine a position of a target pattern printed on a substrate by a lithographic process used to manufacture a device layer on a substrate, the sensor comprising: a broadband radiation source arranged to emit a first beam of radiation having a first wavelength range; an acousto-optical tunable filter comprising an acousto-optical crystal arranged to receive the first beam of radiation, a transducer coupled to the acousto-optical filter and arranged to excite acoustic waves therein, and a beam selecting device arranged to select as an output beam one of a plurality of beams, output by the acousto-optical crystal in response to the first beam and the acoustic waves, as a second beam of radiation having a second wavelength range, the second wavelength range being narrower than the first wavelength range; an optical system comprising a self-referencing interferometer and arranged to direct the second beam of radiation onto the target pattern and to project radiation reflected or scattered by the target pattern onto a detector; and a driver circuit electrically coupled to the transducer and arranged to generate a drive signal therefor, the driver circuit being adapted to control the frequency of the drive signal so as to control the second wavelength range.
 17. An alignment sensor according to claim 16, further comprising a detector; and a beam splitter arranged to divert a predetermined portion of the second beam to the detector, wherein the detector is arranged to detect the intensity of the diverted portion of the second beam and to generate an intensity measurement signal, the driver circuit being responsive to the intensity measurement signal to control the amplitude of the drive signal and thereby the amplitude of the second beam.
 18. An alignment sensor according to claim 16, wherein the drive circuit is arranged to generate a drive signal having a plurality of different frequency components so that the acousto-optical tunable filter is configured to output simultaneously a plurality of second beams of radiation having respectively different wavelength ranges.
 19. An alignment sensor according to claim 18, further comprising a detector; and a beam splitter arranged to divert a predetermined portion of the second beams to the detector, wherein the detector is arranged to detect the intensity of the diverted portion of the second beams of radiation and to generate an intensity measurement signal, the driver circuit being responsive to an intensity measurement signal to control an amplitude of the drive signal and thereby the amplitude of the second beam.
 20. An alignment sensor according to claim 19, wherein the beam splitter is a dichroic beam splitter arranged to divert a predetermined portion of a specific one of the second beams of radiation to the detector.
 21. An alignment sensor according to claim 18, further comprising a plurality of detectors; and a plurality of dichroic beam splitters each arranged to divert a predetermined portion of a respective one of the second beams of radiation to a respective one of the detectors, wherein each of the detectors is arranged to detect an intensity of the diverted portion of the respective second beam of radiation and to generate a respective intensity measurement signal, the driver circuit being responsive to the intensity measurement signal to control the amplitude of the respective frequency component of the drive signal and thereby the amplitude of the respective second beam of radiation. 